Conventional batch chromatography is a separation process which uses a principle of adsorption mechanism. Since it is suitably used for separation with high purity, assays carried out in a laboratory and the like, it is widely used in a separation or purification process of biosynthetic compounds with high purity, fine chemical compounds, food additives and the like. However, such separation processes using batch chromatography have some problems such that it requires a large amount of solvent used as a mobile phase; separation of a component is hardly achieved when the component to be separated has a similar adsorption property; and it is not appropriately used for separation in massive amount and continuous separation.
In order to solve these problems, a true moving bed (TMB) adsorptive separation process has been suggested in literatures such as Korean laid-open patent application No. 2001-51842 and the like. The TMB process introduces the concept of a counter current flow which has been efficiently used in various processes such as heat exchange and extraction, in which a flow which is counter to the flow direction of the mobile phase is applied to the stationery phase, thus when a mixed solution intended to be separated is fed into a column, a component which has stronger adsorption tendency to the stationary phase comes out of the column along with the flow of the stationery phase, and another component which has lower adsorption tendency to the stationary phase comes out of the column along the flow of the mobile phase. Therefore, this process is advantageous in that it is possible to obtain pure substance as long as two components can be separated at each end point of the concentration distribution curves of the two components, although they have not so much different separability. In the meantime, it also has disadvantages such that the amount of a filling material should be increased as compared to the conventional fixed type separation process, and work in normal state is hardly achieved owing to the friction and leakage of the filling material.
For overcoming these problems of the TMB process, a simulated moving bed (SMB) adsorptive separation process has been developed. The SMB process solves the problems related to the flow of a stationery phase in the TMB process, with the simulation of the counterflow of the solid phase by filling and fixing the adsorbent that is a stationery phase into a column and stepwise moving the ports between columns at a certain time interval. Currently, the SMB process is applied to a separating and purifying process of p-xylene from aromatic hydrocarbon mixtures, a separation process of ethyl benzene, a separation process of chiral compounds and the like. One representative SMB process among SMB processes which are commercially practiced is disclosed in U.S. Pat. Nos. 4,326,092 and 5,382,747 applied by UOP LLC, normally referred as “Parex process”.
Parex process is comprised of one or two long adsorption chambers connected in series, wherein the adsorbent chamber is divided into a number of adsorbent beds, normally 12 beds per adsorbent chamber. In a simulated moving bed adsorptive separation process like Parex process, the flow of a stationery phase is not practically realized. Instead, positions of inlet and outlet ports for desorbent, extract, fluid mixture (feed), raffinate and cleaning liquid are moved in the direction of the flow of a mobile phase at a certain time interval of rotary valve rotation, so as to move the columns in the counter direction relative to the flow direction of the mobile phase, with each port as the center, wherein the time interval of the rotary valve rotation is referred as switching time. As such, the virtual flow of the stationery phase can be made to simulate the counterflow to the flow of the mobile phase. The adsorbent used as a stationery phase is filled into the bed.
In Parex process, although each position of ports for desorbent, extract, fluid feed mixture and raffinate cannot be continuously moved, similar effects can be obtained by providing a multiple access line and periodically switching each flow to adjacent line by using a rotary valve at a given time interval of switching time. During the process, a material with lower adsorption in the fluid feed mixture injected through a feed inlet port, comes out though a raffinate outlet port along the mobile phase, and a material with higher adsorption in the fluid feed mixture is adsorbed to each adsorbent bed of the adsorbent chamber. As the column relatively moves at a certain switching time, the adsorbed material can be recovered through the extract outlet port after a certain time elapse.
However, the conventional Parex process for the production of p-xylene has a limit in improving productivity, since the concentration of p-xylene in the feed mixture is not high enough. In order to overcome the setback, a method called selective toluene disproportionation, STDP, has been developed, and in case of a process using a conventional crystallizer, a hybrid method comprising pretreatment in an adsorption tower for increasing the production has also been developed. However, these methods still have limitation in increasing the concentration of p-xylene in the feed mixture for final separation to the desired extent.